Skip fire transition control

ABSTRACT

Methods and arrangements are described for controlling transitions between firing fractions during skip fire operation of an engine in order to help smooth the transitions. Generally, firing fractions transitions are implemented gradually, preferably in a manner that relatively closely tracks manifold filling dynamics. In some embodiments, the commanded firing fraction is altered each firing opportunity. Another approach contemplates altering the commanded firing fraction by substantially the same amount each firing opportunity for at least a portion of the transition. These approaches work particularly well when the commanded firing fraction is provided to a skip fire controller that includes an accumulator functionality that tracks the portion of a firing that has been requested, but not delivered, or vice versa. In various embodiments, commanded firing fraction changes are delayed relative to initiation of the change in throttle position to help compensate for inherent delays associated with changing the manifold air pressure.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 13/654,248 filed Oct. 17, 2012, which claims priority ofProvisional Application Nos. 61/548,187 filed Oct. 17, 2011 and61/640,646 filed Apr. 30, 2012. This application claims priority of U.S.Provisional Application No. 62/053,351 filed Sep. 22, 2014. Thisapplication is also a Continuation of International Application No.PCT/US15/50181, filed on Sep. 15, 2015. All of these referenced priorityapplications are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to methods and arrangements forcontrolling transitions between firing fractions during skip fireoperation of an engine.

BACKGROUND

Fuel efficiency of many types of internal combustion engines can besubstantially improved by varying the displacement of the engine. Thisallows for the full torque to be available when required, yet cansignificantly reduce pumping losses and improve thermodynamic efficiencythrough the use of a smaller displacement when full torque is notrequired. The most common method of varying the displacement today isdeactivating a group of cylinders substantially simultaneously. In thisapproach no fuel is delivered to the deactivated cylinders and theirassociated intake and exhaust valves are kept closed as long as thecylinders remain deactivated. For example, an 8 cylinder variabledisplacement engine may deactivate half of the cylinders (i.e. 4cylinders) so that it is operating using only the remaining 4 cylinders.Commercially available variable displacement engines available todaytypically support only two or at most three fixed mode displacements.

Another engine control approach that varies the effective displacementof an engine is referred to as “skip fire” engine control. In general,skip fire engine control contemplates selectively skipping the firing ofcertain cylinders during selected firing opportunities. Thus, aparticular cylinder may be fired during one engine cycle and then may beskipped during the next engine cycle and then selectively skipped orfired during the next. In this manner, even finer control of theeffective engine displacement is possible. For example, firing everythird cylinder in a 4 cylinder engine would provide an effectivedisplacement of ⅓^(rd) of the full engine displacement, which is afractional displacement that is not obtainable by simply deactivating aset of cylinders. Conceptually, virtually any effective displacement canbe obtained using skip fire control, although in practice mostimplementations restrict operation to a set of available firingfractions, sequences or patterns. The applicant has filed a number ofpatents describing various approaches to skip fire control.

A known characteristic of skip fire control is that engines operatingunder skip fire control tend to have less desirable noise, vibration andharshness (NVH) characteristics than “normal”, all-cylinder operation ofan engine. Thus, there are continuing efforts to develop techniques andmechanisms that can help reduce NVH concerns during skip operation whilestill maintaining some of its benefits. Typically, the available skipfire firing fractions/sequences/patterns are chosen at least in partbased on their preferred NVH characteristics. While this reduces NVHwhile operating at these available firing fractions, NVH issues canarise during transitions between different firing fractions. The presentapplication describes techniques that can help manage NVH concerns whiledelivering the desired performance during transitions between differentfiring fractions.

SUMMARY

A variety of methods and arrangements are described for controllingtransitions between firing fractions during skip fire operation of anengine in order to help reduce undesirable NVH consequences andotherwise smooth the transitions. In general, firing fractionstransitions are implemented gradually, preferably in a manner thatrelatively closely tracks manifold filling dynamics.

In some preferred implementations the commanded firing fraction isaltered each firing opportunity. Another described approach contemplatesaltering the commanded firing fraction by substantially the same amounteach firing opportunity for at least a portion of the transition. Theseapproaches work particularly well when the commanded firing fraction isprovided to a skip fire firing timing determining module that includesan accumulator functionality that tracks a portion of a firing that hasbeen requested, but not delivered, or that has been delivered, but notrequested.

In various embodiments, commanded firing fraction changes are delayedrelative to initiation of the change in throttle position to helpcompensate for inherent delays associated with changing the manifold airpressure.

In some implementations, the commanded firing fraction is altered in amanner such that a product of the skipping fraction and the intakemanifold pressure remains substantially constant throughout thetransition.

In some implementations, the commanded firing fraction is changed eachfiring opportunity using a linear slew rate such that the amount thatthe commanded firing fraction is changed each firing opportunity is thesame throughout the transition. The actual slew rate that is appropriatefor any particular transition may vary based on factors such as themagnitude of the desired firing fraction change and various engineoperating parameters such as engine speed, etc.

A variety of other engine control techniques including spark retard,feed forward throttle control, feed forward camshaft control, pumpingair through skipped cylinders and others may be used to help furthersmooth the transition.

Techniques are also described for handling situations in which thetarget firing fraction changes in the middle of a transition.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and the advantages thereof, may best be understood byreference to the following description taken in conjunction with theaccompanying drawings in which:

FIG. 1 is a functional block diagram of a skip fire controller inaccordance with one embodiment of the present invention.

FIG. 2 schematically illustrates the impact of pattern vibration andtorque mismatch vibration during a transition.

FIGS. 3(a)-3(d) are a set of graphs showing the requested and adjustedfiring fractions, throttle position, intake manifold pressure andoverall engine torque during a representative intended constant torquetransition between a firing fraction of ⅓^(rd) and a firing fraction of⅔^(rd) utilizing a first order band pass filter to smooth thetransition.

FIGS. 4(a)-4(d) are a set of graphs showing two types of adjusted firingfractions, throttle position, intake manifold pressure and overallengine torque in a representative intended constant torque transitionbetween a firing fraction of ⅓^(rd) and a firing fraction of ⅔^(rd)utilizing a delayed linear slew in accordance with one embodiment of theinvention.

FIG. 5 is a graph showing the firing fraction (Y-axis) as a function oftime (X-axis) during a representative interrupted transition where asecond firing fraction transition is requested while a first transitionis in progress.

FIGS. 6(a)-6(d) are a set of graphs showing the adjusted firingfraction, intake manifold pressure, spark timing, and overall enginetorque in a representative intended constant torque transition between afiring fraction of ⅓^(rd) and a firing fraction of ⅔^(rd) utilizing adelayed linear slew in accordance with one embodiment of the invention.

FIG. 7 is a graph showing the adjusted skipping fraction and intakemanifold pressure in a representative intended constant torquetransition between a firing fraction of ⅓^(rd) and a firing fraction of⅔^(rd) utilizing a delayed linear slew and pumping air throughdeactivated cylinders in accordance with one embodiment of theinvention.

FIG. 8 is an exemplary look up table of firing fraction slew rates fordifferent initial and target firing fractions in accordance with oneembodiment of the invention.

In the drawings, like reference numerals are sometimes used to designatelike structural elements. It should also be appreciated that thedepictions in the figures are diagrammatic and not to scale.

DETAILED DESCRIPTION

When a limited set of firing fractions are available, transitionsbetween different firing fractions typically also involve the adjustmentof selected engine operating parameters. This is because at anyparticular firing density, there will be associated operating parameters(e.g., air charge, spark timing, etc.) that are appropriate toefficiently deliver the desired engine output. Therefore, when a changeis made in the firing density, it is typically desirable tosubstantially simultaneously adjust selected engine operating parametersso that the desired engine output is maintained at the new firingfraction. Without such an adjustment, operating at the same enginesettings would typically result in the generation of more torque thandesired when the firing density is increased, and less torque thandesired when the firing density is reduced.

From a control standpoint, the firing density can be changed veryquickly by simply altering the selection of the specific cylinders to befired—however corresponding changes in the air charge tend to berealized more slowly due to the latencies inherent in filling oremptying the intake manifold. This is particularly noticeable when thedesired firing fraction changes significantly, as for example from ½ to1 or from ⅓ to ⅔, which require correspondingly large changes in aircharge/manifold pressure. Generally, any mismatch between the firingdensity and the targeted cylinder air charge during a transition, willresult in a low frequency torque disturbance (unless otherwisecompensated for), which may be perceived as NVH. If the mismatch wouldresult in a torque surge, then the spark timing can be retarded tomaintain the desired torque. However, an undesirable side effect ofretarding spark to reduce engine output is that retarding spark willgenerally reduce fuel efficiency. Also, excessive spark retard will leadto misfires further reducing efficiency and potentially adverselyaffecting the engine.

The torque mismatch problem can also be mitigated somewhat by slowingthe transition between firing fractions. Slowing the transition allowschanges in the firing density to more closely track changes in theintake manifold pressure. However, even if the air/torque is preciselymatched with changes in the firing fraction, any change from one firingdensity to another will cause a low frequency vibration, since theintermediate firing fractions have undesirable firing patterns. Slowingthe transition tends to exacerbate these types of disturbances. Theperceived total NVH may be thought of as the summation of these twoeffects. Components from the transition firing patterns, NVH_(pattern),and from the transition torque mismatch, NVH_(mismatch), can be thoughtto add to form a total NVH_(total), i.e.NVH_(total)=NVH_(pattern)+NVH_(mismatch). FIG. 2, which plots NVH as afunction of the transition time, schematically illustrates thesituation. For short transition times, the NVH_(pattern) curve 170 islow and the NVH_(mismatch) curve 172 is high. For long transition times,the situation is reversed. The NVH_(total) curve 174 shows a minimumvalue, often in the vicinity of 200 milliseconds as shown in FIG. 2.Thus it is often desirable to have a transition length in the vicinityof 200 msec, for example from about 150 msec to about 300 msec. Itshould be appreciated that some transitions may be longer or shorter andthat in some cases the total NVH may not strictly be the sum of patternand mismatch NVH, but such a conceptualization is generallyqualitatively accurate.

The applicant has previously described a variety of skip firecontrollers. A skip fire controller 10 suitable for implementing thepresent invention is functionally illustrated in FIG. 1. The illustratedskip fire controller 10 includes a torque calculator 20, a firingfraction determining unit 30, a transition adjustment unit 40, a firingtiming determination unit 50, and a power train parameter adjustingmodule 60. For the purposes of illustration, skip fire controller 10 isshown separately from engine control unit (ECU) 70. However, it shouldbe appreciated that in many embodiments the functionality of the skipfire controller 10 may be incorporated into the ECU 70. Indeedincorporation of the skip fire controller into an ECU or power traincontrol unit is expected to be the most common implementation.

The torque calculator 20 is arranged to determine the desired enginetorque at any given time based on a number of inputs. The torquecalculator outputs a requested torque 21 to the firing fractiondetermining unit 30. The firing fraction determining unit 30 is arrangedto determine a firing fraction that is suitable for delivering thedesired torque based on the current operating conditions and outputs afiring fraction 33 that is appropriate for delivering the desiredtorque. The firing timing determining unit 50 is responsible fordetermining a firing sequence that delivers the desired firing fraction.The firing sequence can be determined using any suitable approach. Insome preferred implementations, the firing decisions are madedynamically on an individual firing opportunity by firing opportunitybasis which allows desired changes to be implemented very quickly. Avariety of firing timing determining units that are well suited fordetermining appropriate firing sequence based on potentially timevarying requested firing fraction or engine outputs have been previouslydescribed by the Applicant. Many such firing timing determining unitsare based on a sigma delta converter which is well suited for makingfiring decisions on a firing opportunity by firing opportunity basis. Insome cases the initial accumulator value in the sigma delta convertermay be set at the beginning of a transition in order to generate afiring pattern with low NVH during the transition. In otherimplementations, pattern generators or predefined patterns may be usedto facilitate delivery of the desired firing fraction.

The torque calculator 20 receives a number of inputs that may influenceor dictate the desired engine torque at any time. In automotiveapplications, one of the primary inputs to the torque calculator is theaccelerator pedal position (APP) signal 24 which indicates the positionof the accelerator pedal. In some implementations the accelerator pedalposition signal is received directly from an accelerator pedal positionsensor (not shown) while in others an optional preprocessor 22 maymodify the accelerator pedal signal prior to delivery to the skip firecontroller 10. Other primary inputs may come from other functionalblocks such as a cruise controller (CCS command 26), the transmissioncontroller (AT command 27), a traction control unit (TCU command 28),etc. There are also a number of factors such as engine speed that mayinfluence the torque calculation. When such factors are utilized in thetorque calculations, the appropriate inputs, such as engine speed (RPMsignal 29) are also provided or are obtainable by the torque calculatoras necessary.

Further, in some embodiments, it may be desirable to account forenergy/torque losses in the drive train and/or the energy/torquerequired to drive engine accessories, such as the air conditioner,alternators/generator, power steering pump, water pumps, vacuum pumpsand/or any combination of these and other components. In suchembodiments, the torque calculator may be arranged to either calculatesuch values or to receive an indication of the associated losses so thatthey can be appropriately considered during the desired torquecalculation.

The nature of the torque calculation will vary with the operationalstate of the vehicle. For example, during normal operation, the desiredtorque may be based primarily on the driver's input, which may bereflected by the accelerator pedal position signal 24. When operatingunder cruise control, the desired torque may be based primarily on theinput from a cruise controller. When a transmission shift is imminent, atransmission shifting torque calculation may be used to determine thedesired torque during the shifting operation. When a traction controlleror the like indicates a potential loss of traction event, a tractioncontrol algorithm may be used to determine the desired torque asappropriate to handle the event. In some circumstances, depression of abrake pedal may invoke specific engine torque control. When other eventsoccur that require measured control of the engine output, appropriatecontrol algorithms or logic may be used to determine the desired torquethroughout such events. In any of these situations, the required torquedeterminations may be made in any manner deemed appropriate for theparticular situation. For example, the appropriate torque determinationsmay be made algorithmically, using lookup tables based on currentoperating parameters, using appropriate logic, using set values, usingstored profiles, using any combinations of the foregoing and/or usingany other suitable approach. The torque calculations for specificapplications may be made by the torque calculator itself, or may be madeby other components (within or outside the ECU) and simply reported tothe torque calculator for implementation.

The firing fraction determining unit 30 receives requested torque signal21 from the torque calculator 20 and other inputs such as engine speedand various power train operating parameters and/or environmentalconditions that are useful in determining an appropriate operationalfiring fraction 33 to deliver the requested torque under the currentconditions. The firing fraction is indicative of the fraction orpercentage of firings that are to be used to deliver the desired output.Often, the firing fraction determining unit will be constrained to alimited set of available firing fractions, patterns or sequences thathave been selected based at least in part on their relatively moredesirable NVH characteristics (collectively sometimes referred to hereingenerically as the set of available firing fractions). There are anumber of factors that may influence the set of available firingfractions. These typically include the requested torque, cylinder load,engine speed (e.g. RPM) and current transmission gear. They maypotentially also include various environmental conditions such asambient pressure or temperature and/or other selected power trainparameters. The firing fraction determining unit 30 is arranged toselect the desired operational firing fraction 33 based on such factorsand/or any other factors that the skip fire controller designer mayconsider important. By way of example, a few suitable firing fractiondetermining units are described in co-pending application Ser. Nos.13/654,244; 13/654,248, 13/963,686 and 14/638,908, each of which areincorporated herein by reference.

The number of available firing fractions/patterns and the operatingconditions during which they may be used may be widely varied based onvarious design goals and NVH considerations. In one particular example,the firing fraction determining unit may be arranged to limit availablefiring fractions to a set of 29 possible operational firingfractions—each of which is a fraction having a denominator of 9 orless—i.e., 0, 1/9, ⅛, 1/7, ⅙, ⅕, 2/9, ¼, 2/7, ⅓, ⅜, ⅖, 3/7, 4/9, ½, 5/9,4/7, ⅗, ⅝, ⅔, 5/7, ¾, 7/9, ⅘, ⅚, 6/7, ⅞, 8/9 and 1. However, at certain(indeed most) operation conditions, the set of available firing fractionmay be reduced and sometimes the available set is greatly reduced. Ingeneral, the set of available firing fractions tends to be smaller inlower gears and at lower engine speeds. For example, there may beoperating ranges (e.g. near idle and/or in first gear) where the set ofavailable firing fractions is limited to just two availablefractions—(e.g., ½ or 1) or to just 4 possible firing fractions—e.g., ⅓,½, ⅔ and 1. Of course, in other embodiments, the permissible firingfractions/patterns for different operating conditions may be widelyvaried.

Since the available set of firing fractions is limited, various powertrain operating parameters such as mass air charge (MAC) and/or sparktiming will typically need to be varied to insure that the actual engineoutput matches the desired output. In the illustrated embodiment, apower train parameter adjusting module 60 is provided that cooperateswith the firing fraction calculator 30. The power train parameteradjusting module 60 directs the ECU 70 to set selected power trainparameters appropriately to insure that the actual engine outputsubstantially equals the requested engine output at the commanded firingfraction. By way of example, the power train parameter adjusting module60 may be responsible for determining the desired MAC, spark timingand/or other engine settings that are desirable to help ensure that theactual engine output matches the requested engine output. Although thepowertrain parameter adjusting module 60 is illustrated as a separatecomponent, it is often implemented as a part of engine control unit 70.Of course, in other embodiments, the power train parameter adjustingmodule 60 may be arranged to directly control various engine settings.

The firing timing determining module 50 is arranged to issue a sequenceof firing commands 52 that cause the engine to deliver the percentage offirings dictated by a commanded firing fraction 48. The firing timingdetermining module 50 may take a wide variety of different forms. By wayof example, sigma delta convertors work well as the firing timingdetermining module 50. A number of the assignee's patents and patentapplications describe various suitable firing timing determiningmodules, including a wide variety of different sigma delta basedconverters that work well as the firing timing determining module. See,e.g., U.S. Pat. Nos. 7,577,511, 7,849,835, 7,886,715, 7,954,474,8,099,224, 8,131,445, 8,131,447, 8,839,766 and pending application Ser.No. 13/774,134 filed Feb. 22, 2013. The sequence of firing commands(sometimes referred to as a drive pulse signal 52) outputted by thefiring timing determining module 50 may be passed to an engine controlunit (ECU) 70 or another module such as a combustion controller (notshown in FIG. 1) which orchestrates the actual firings. A significantadvantage of using a sigma delta converter or an analogous structure isthat it inherently includes an accumulator function that tracks theportion of firing that have been requested but not yet delivered. Suchan arrangement helps smooth transitions by accounting for the effects ofprevious fire/no fire decisions.

As suggested above, abrupt transitions between firing fractions can leadto undesirable vibrations and/or torque surges or dips, i.e. undesirableNVH_(total) as discussed relative to FIG. 2. The torque surge/dip arisessince often the change in torque request, at least during thetransition, is smaller than the change in the firing fraction. Thefiring fraction change would thus cause the engine toovershoot/undershoot the requested torque level. Therefore, in theembodiment illustrated in FIG. 1, transition adjustment unit 40 isarranged to help mitigate vibrations and torque surges/dips associatedwith step changes in the requested firing fraction 33. When a stepchange in requested firing fraction occurs, the transition adjustmentunit 40 has the effect of spreading the change in firing fraction over ashort period. This “spreading” (which may include a brief delay) canhelp smooth transitions between different commanded firing fractions andcan help compensate for various delays associated with manifold filling.These may include mechanical delays in the changing of the engineparameters and/or inertial type manifold filling/emptying delays. Whenthe requested firing fraction is at steady state, the commanded firingfraction 48 is the same as the requested firing fraction 33. However,when a transition occurs, the commanded firing fraction 48 iseffectively ramped from the previous requested firing fraction to thetarget firing fraction.

If the nature of the transition is such that the transition adjustmentunit imposed delays are acceptable, smoother operation can be obtainedby using such an arrangement. However, if the nature of the transitionis such that a quicker response is desired (as for example, when thedriver stomps on the accelerator pedal or during traction controlevents), it may be desirable to bypass or modify the settings of thetransition adjustment unit 40 to provide a quicker response. Therefore,some implementations incorporate separate “fast path” and “slow path”approaches for managing firing fraction change requests. In suchapplications, the transition adjustment unit can be bypassed for “fastpath” responses and used in “slow path” changes. More generally, thetransition adjustment unit 40 characteristics may vary depending oninputs governing the desired transition, i.e. the desired firingfraction slew rate may vary with the rate of change and/or magnitude ofchange of the accelerator pedal position.

To account for some of the intake manifold filling dynamics describedabove, the applicant has previously proposed using a filter at thelocation of transition adjustment unit 40 that roughly mimics the airfilling dynamics to smooth transitions between firing fractions. By wayof example, such an approach is described in U.S. patent applicationSer. Nos. 13/654,244 and 13/654,248 which are incorporated herein byreference. In general, the requested firing fraction is passed throughone or two filters before reaching the firing timing determining unit sothat step changes in the requested firing fraction are more graduallypresented to the firing timing determining module. Another transitionmanagement approach is described in co-pending application Ser. No.14/203,444.

The filtering approach described in the incorporated patents works wellto help mitigate vibrations. The Applicant has found that even betterresults can sometimes be obtained utilizing some of the slew rate basedtechniques described below.

In one aspect a designated firing fraction slew rate is used to helpsmooth the transition between firing fractions. In some implementationsa brief delay is also included before initiating the slewed transition.The appropriate slew rate for any transition can depend on a number ofoperating parameters including current engine speed, intake/exhaustvalve timing, torque demand, starting firing fraction and target firingfraction, the mass air charge, etc. The slew rate may also depend onvehicle parameters, such as manifold size, acoustic and vibrationtransfer paths between NVH sources and the cabin occupants, and vehiclestyle, i.e. sedan, sports car, luxury car, etc. By way of example,linear slew rates on the order of 1-5 percent of the firing fraction perfiring opportunity work well in many applications. A linear slew rate of2% will make a transition from a firing fraction of 0 to 1 over thecourse of 50 firing opportunities from the time that the transitionbegins, which would be just over 6 engine cycles in an eight cylinderengine. A slew rate of 1% will take twice as long to transition, while aslew rate of 4% would result in transitions that take half as long. Byway of example, if a transition is being made from a firing fraction of½ to a firing fraction of 1, at a slew rate of 2% would suggest that thecommanded firing fraction for the first firing opportunity after anyimposed delay would be 52%, the commanded firing fraction for the secondfiring opportunity would be 54% and so on until the desired firingfraction of 1 is obtained. Of course, the slew rate in other instanceswould vary with the initial and target firing fractions.

An appropriate slew rate can be determined by looking at the manifoldpressure response to throttle movement during a transition. By measuringintake manifold absolute pressure (MAP) as a function of time adetermination of how fast the engine can pump down the manifold pressurecan be made. Engines with a small intake manifold or ones that operateat a high engine speed, may use a higher slew rate than engines with alarge intake manifold or when operating at low RPM. Generally, an intakemanifold will fill faster than it will empty. Increases in firingfraction typically require emptying the intake manifold and decreases infiring fraction typically require filling the intake manifold to avoid atorque bump or dip. Therefore, it is often desirable to utilize a slowerslew rate for increases in firing fraction (which typically correspondto lower desired air charges/manifold pressures) than for decreases infiring fraction (which typically correspond to higher desired aircharges/manifold pressures). As suggested above, a variety of otherfactors including engine speed, intake/exhaust valve timing, current aircharges, the current and target firing fractions can also influence themanifold filling/emptying dynamics so the transition adjustment unit 40may be arranged to set the appropriate slew rate based in part on any ofthese—or other appropriate factors.

An appropriate slew rate may be determined using a variety oftechniques. In some cases a look-up table may be used to determine theappropriate slew rate between initial and target firing fractions. FIG.8 illustrates such a table 800, where the slew rate is expressed in thepercentage change in firing fraction per firing opportunity. Of course,the slew rate could be expressed in terms of other variables, such astime, crank angle, etc. The entries in table 800 should be consideredexemplary only and may in practice be different from those given in FIG.8. Table 800 lists 29 possible first, initial firing fractions and 29possible second, target firing fractions. The central diagonal 880 showsno entries, since it corresponds to the initial and target firingfractions being equal, i.e. no transition. The entries above diagonal880 correspond to increases in firing fraction and the entries below thediagonal correspond to decreases in firing fraction. As previouslymentioned generally the intake manifold fills more quickly than itempties, so the slew rates above the diagonal are generally smaller thanthose below the diagonal. Another feature evident in the table isincreasing slew rates for transitions having large changes in firingfraction. This helps to reduce the transition time minimizingNVH_(pattern). A further feature is that some transitions have such asmall change in firing fraction that the slew rate can be set to 100%,i.e. a step function change in the firing fraction. The firing fractionvalues derived from the slew rate in table 800 and initial firingfraction may be used as an input to a delta sigma converter to determinea firing sequence. In other embodiments an appropriate firing sequencecan be determined directly using a look up table of various firingsequences that can be used to transition between different firingfractions.

The actual slew rated used during any given transition may be modifiedfrom those given in look up table 800 based on engine operationalconditions and driver input. For example, if a driver rapidly depressesor releases the accelerator pedal the transition actual slew rates maybe increased to make the vehicle more responsive. In some cases thetable may be not used and the firing fraction may immediately change toits target value. This will likely have significant NVH consequences,but large, fast accelerator pedal position changes may indicate a safetyissue, which takes priority over NVH concerns. High engine speeds resultin more firing opportunities within a given time window. Thus it ispossible to have a transition time in the vicinity of 200 ms, whilehaving a slower actual slew rate. Changes in intake/exhaust valve timingmay influence the amount of air inducted during each firing event, thusimpacting the rate of intake manifold filling/emptying. As such, theactual slew rate may be used to modify values from those shown in table800 based on valve timing. Engine speed also influences the amount ofair inducted by a cylinder, so it may also impact the actual slew rate.It should be appreciated that rather than having a single twodimensional look up table similar to that shown in FIG. 8 and modifyingactual slew rate values, higher dimensionality tables could be usedwhich incorporate additional variables as indices, i.e. valve timing,engine speed, etc.

As suggested above, a brief delay may also be imposed before beginningthe transition. The length of the delay can vary based on the nature ofthe change and design choices for a particular engine which may involvea number of tradeoffs in terms of desired responsiveness, NVHconsiderations and design simplicity. By way of example, delays on theorder of 1 to 10 firing opportunities have been found to work well invarious implementations. Depending on the engine speed and number ofengine cylinders this delay can be several milliseconds to on the orderof 100 milliseconds. Alternatively, in some cases it may be desirable todelay motion of the throttle and initiation of a change in the manifoldabsolute pressure until after a transition in the firing fraction hasbegun. This type of delay may be particularly advantageous intransitions from higher to lower firing fractions, where spark timingadjustment can be used to decrease the per cylinder torque output. Also,in some cylinder activation/deactivation methods there may be a delaybetween making a decision to change the firing fraction and theimplementation of the decision. Thus, the manifold pressure may alreadybe changing by the time the actual change in firing fraction begins. Theappropriate value for the delay may be found in a look up table similarto that shown in FIG. 8, where the entries now correspond to the delayassociated with various transitions. The actual delay values used may bemodified from those listed in the table in a manner similar to thatdescribed relative to FIG. 8. Alternatively, the delay values may not belisted in a look up table, but may be determined based on engineparameters and operational conditions.

A very significant challenge in control occurs when it is desired tochange the firing fraction while holding the produced engine torqueconstant. This may be considered a limiting case of small changes intorque request causing a change in the required firing fraction. FIG.3(a)-3(d) depicts, in simplified form, engine operation in such anidealized case. FIG. 3(a)-3(d) illustrate the requested and adjustedfiring fractions, throttle position, intake manifold absolute pressure(MAP), and overall engine torque output during an intended constanttorque transition between a firing fraction of ⅓^(rd) and a firingfraction of ⅔^(rd). In FIG. 3(a) the requested firing fraction 210 isshown as a step function between the initial value of ⅓^(rd) and thefinal value of ⅔^(rd). The step occurs at a time t₁, which can be set tozero and defines the start of the transition. The adjusted firingfraction 212 is shown tracking the requested firing fraction until thetime t₁ and then following a trajectory described by a first order lowpass filter. At a time at the end of the transition, t_(tr), therequested and adjusted firing fractions are again equal.

It should be appreciated that the transition time, t_(tr), can varydepending on a variety of conditions such as torque request, enginespeed, transmission gear, and cylinder load in the initial and finalstates. The transition time is generally chosen to provide acceptableNVH performance as discussed in relation to FIG. 2.

FIG. 3(b) shows the response of the throttle blade position 220 versustime. The first initial throttle position 220 a remains constant untilthe start of the transition at time t₁. A feed forward control algorithmmay be used to control the throttle blade position, since will reducethe overall transition time reducing NVH_(pattern). Since in thistransition the target MAP is lower than the initial MAP, the throttlewill close during the transition to help reduce the MAP. The throttlemoves to a closed position over a time period, T_(th). The duration ofT_(th) is defined at least in part by the delay in processing therequested firing fraction signal into a new throttle position and thetime necessary to physically move the throttle blade. This time can bequite small, approximately 20 msec, on the order of 1 or several firingopportunities. The throttle blade stays in the closed position over amajority of the transition. It moves to its final target position 220 bnear the end of the transition. It stays in a substantially constantposition throughout the remainder of the transition. In FIG. 3b thesecond target throttle position is more fully open than the firstinitial throttle position. This may seem counter intuitive, since thesecond target MAP is lower than the first initial MAP; however, sinceFIGS. 3a-d depict an idealized constant torque transition the engine airinduction should be similar between the initial and target operatingconditions. Difference in the air induction level will arise fromdifferences in engine efficiency. At the second target condition thepumping losses are greater, since MAP is lower, and thus the enginewould need more air to produce the same torque. Other factors thatinfluence engine efficiency include spark timing, intake/exhaust valvetiming and lift, and cylinder load. In general these variables may causemore or less efficient operation in the initial or target operatingconditions, thus the target throttle position may be more or less openthan the initial throttle position.

It should be appreciated that the throttle trajectory depicted in FIG.3(b) is a representative idealized throttle trajectory. In practiceother types of throttle trajectories may be used such as closed loopcontrol of MAP, closed loop MAP with additional feed forward throttlecontrol. The position of the throttle during the transition can vary andthe final throttle position may be higher, similar to or lower than theinitial value.

FIG. 3(c) shows the MAP and skipping fraction as a function of time. Theskipping fraction is defined as one minus the firing fraction. Therequested skipping fraction 236 and the resultant requested MAP 230 andthe adjusted skipping fraction 238 and resultant adjusted MAP 231 areboth shown. The two manifold pressures 230 and 231 and skippingfractions 236 and 238 are substantially constant until the beginning ofthe transition at time t₁. The requested MAP 230 begins to drop inresponse of the closing of the throttle plate and the removal of airfrom the intake manifold by induction into the cylinders. As previouslydescribed the MAP 230 response is relatively slow because of thefilling/emptying dynamics of the intake manifold. The requested skippingfraction 236 mirrors the response of the firing fraction 210 shown inFIG. 3(a) and has a step function drop at the beginning of thetransition. The adjusted skipping fraction 238 has a more gradualtransition. The more gradual transition associated with the adjustedskipping fraction 238 results in a slower pump down of the intakemanifold resulting in the adjusted MAP 231 transitioning more slowlythan the requested MAP 230. If other engine parameters are fixed, theproduct of the MAP and firing fraction is substantially proportional tothe engine torque output. For a constant torque output this implies thatthat the MAP and skipping fraction should track each other, i.e. theirproduct should be substantially constant through the transition. Theextent of MAP and skipping fraction divergence is indicative of thetorque mismatch. The areas 234 and 235 define the mismatch associatedwith the adjusted firing fraction and are proportional to the excesstorque generated by the adjusted firing fraction 212. Similarly the sumof area 232 and 234 is proportional to the excess torque generated bythe requested firing fraction 210.

The torque surge associated with this mismatch between the firingfraction and MAP is more clearly shown in FIG. 3(d). The torque surgethat would occur based on an immediate transition of the firing fractionto the target firing fraction is illustrated by curve 240. By contrastthe torque surge generated through the use of the adjusted firingfraction is illustrated by curve 242. It can be seen that the overalltorque surge is significantly reduced through the use of a filter tosmooth the transition, although the duration of the surge may beextended somewhat due to the fact that the intake manifold pressure isnot pumped down as quickly. The net torque surge is the integral of thetorque mismatch over the duration of the mismatch. Although using afirst order filter to smooth the transition between firing fractions cansignificantly reduce the torque surge/sag associated with a transition,it can be difficult to define a filter (or set of filters) that workwell over a wide variety of operating conditions and over the entirerange of possible firing fraction changes. One reason for this is thatchanges arising from a linear filter are proportional to change in thefraction while enduring for the same amount of time. So for example, achange from ½ to 1 will be twice as large as a change from ½ to ¾, butwill occur in the same time period. Manifold dynamics, especiallyemptying, tend to be different. Instead of changing faster with largerchanges, the rate of change is similar, but the duration of the changeis longer. Another aspect of a linear filter is that the response to astep change, like that seen when the firing fraction changes, has thegreatest change in output at the time of the step, and a decreasingamount of change with each succeeding step. Not only does this not matchthe physical behavior, it also worsens the consequences of misalignmentof the start of the firing fraction transition with the physicalbehavior of the manifold.

A linear slew rate transition management strategy may be implemented inplace of the prior art filter schemes to help further reduce the torquesurge/dip and vibration in a number of applications. Some of thepotential advantages of this approach will be schematically describedwith reference to FIG. 4. More specifically, FIGS. 4(a)-4(d) generallyshow the same type of information as shown in FIGS. 3(a)-3(d), exceptthat the figures diagrammatically compare the exemplary use of a delayedlinear slew rate to an exemplary first order filter. It should beappreciated that the figures are diagrammatic in nature and are intendedto illustrate the concepts rather than reflect data from a specific testsince the nature of the curves in practice will necessarily dependheavily on a number of variables including engine speed and operatingconditions, the nature of the filter's transfer function, the designatedslew rate, the delay incorporated, etc.

Initially, FIG. 4(a) compares a filtered firing fraction change 212 to afiring fraction change trajectory 310 that might be seen when using adelayed linear slew rate. The filtered firing fraction change 212 isidentical to that shown in FIG. 3(a). In firing fraction trajectory 310,the commanded firing fraction 310 remains at the original firingfraction from t₁ (the start of the transition) for a designated delayperiod T_(D). The time delay, T_(D), may be larger or smaller than thethrottle transition time t_(th). The appropriate time delay T_(D) canvary based on a number of factors including manifold filling/emptyingdynamics, throttle response time, etc. After the delay, the firingfraction 310 rises linearly until it reaches the target firing fraction.

FIG. 4(b) shows the response of the throttle position 220 versus time.This figure is identical to FIG. 3(b) and the description will not berepeated. FIG. 4(c) shows the MAP and skipping fraction as a function oftime. In FIG. 4(c) the filtered skipping fraction 238 and resultant MAP231 are identical to that shown in FIG. 3(c). The torque mismatchassociated with the filtered firing fraction is depicted by the areas334 and 332 a. This area is equivalent to that depicted in FIG. 3(c).The delayed linear slew skipping fraction 330 mirrors the changes in thefiring fraction 310. The torque mismatch associated with the delayedlinear slew is depicted by areas 332 a and 332 b. Area 332 a isassociated with a torque surge, while area 332 b is associated with atorque lag. For clarity this figure assumes that the MAP 231 associatedwith both the filtered and delayed linear slew are equal, whereas inpractice they will be somewhat different due to the different pump downrates associated with the different firing fraction trajectories.

FIG. 4(d) shows the resultant torque between the two cases. Curve 242illustrates the torque produced using the filtered firing fraction andis identical to that shown in FIG. 3(d). Curve 340 illustrates thetorque produced using a delayed linear slew rate change in the firingfraction. The torque mismatch associated with the delayed linear slewrate change in firing fraction is significantly less than thatassociated with the filtered changed demonstrating the advantage of thiscontrol method.

When a linear slew rate is utilized, both the slope of the linear slewand the length of the delay (if any) will each have a direct impact onboth the magnitude and sense of the torque mismatch. Various enginecharacteristics and operating parameters will also affect the torquemismatch (e.g., engine speed, manifold characteristics, spark timing,valve timing, valve lift, air/fuel stoichiometry, etc.). When the linearslew rate is selected such that it closely approximates the manifoldfilling dynamics, the torque surge or lag associated with a transitioncan be significantly reduced. Indeed, it has been observed that in manytransitions, a properly selected linear slew rate can more closely trackthe manifold filling dynamics than the described first order filter. Itshould, however, be appreciated that the magnitude of a surge or lag canvary significantly based on how closely (or how poorly) the selectedslew rate approximates the manifold filling dynamics.

There are times when a transition may be interrupted by a new targetfiring fraction request. That is, in the middle of a transition from afirst firing fraction to a second firing fraction, there may be timeswhen a second change request is made. In such a case, the transitionunit may begin implementing the second change from the current staterather than waiting for the first transition to be completed. Forexample, consider a circumstance in which the firing fractiondetermining unit 30 requests a change to a firing fraction of ⅞ whilethe transition adjustment unit 40 is in the middle of a transition froma firing fraction of ⅕ to ⅜. Such a situation is graphically representedin FIG. 5 which is a graph showing the firing fraction (Y-axis) as afunction of time (X-axis). In the illustrated embodiment, the engine isinitially operating at a firing fraction of ⅕ as represented by linesegment “a”. At time t₁, the requested firing fraction increases to ⅜.After the designated delay (T_(D) in the illustrated embodiment), thetransition towards the ⅜ firing fraction begins at time t₂ using thedesignated slew rate (1% in this case, see FIG. 8). At time t₃ a changerequest to ⅞ is received—even though the transition to ⅜ has not yetbeen completed. The transition towards the ⅜ firing fraction continuesduring the designated delay, but once the designated delay is completedat t₄, the transition to a firing fraction of ⅞ begins at the designatedslew rate. The delay T_(D1) between the decision to change the targetfiring fraction, occurring at t₃, and the change in slew rate, occurringat t₄, may be different than the delay associated with the beginning ofthe transition T_(D). In some cases the delay T_(D1) may be zero,although in many cases there is an inherent delay between a firingdecision and the implementation of that decision due to the cylinderactivation/deactivation mechanism. In the illustrated embodiment, thedesignated delays are the same for both changes, although that is not arequirement. In this case the slew rate associated with the transitionbetween ⅕ and ⅞ is 2% (see FIG. 2) There is an inflection point in thefiring fraction slew rate at time t₄. Often larger changes between theinitial and final firing fraction will result in a faster slew rate toavoid excessive pattern induced NVH. In this example the magnitude ofthe change from the current firing fraction to ⅞ is greater than themagnitude of original change from ⅕ to ⅜—so the slew rate is increased.In some cases the relative slew rate, i.e. the ratio of the slew rate tothe total change in firing fraction may be held approximately constantthrough the transition. In other cases it may be desirable to leave theslew rate fixed throughout the entire transition. For reference, thedashed line labeled “b” shows the completion of the change to a firingfraction of ⅖ as if the second change had not been requested. Althoughonly a single mid-transition change is represented, it should beappreciated that the same principles can be applied to implement anyfurther changes that are requested during a transition. These caninclude both increases and decreases in the requested firing fraction,multiple sequential change requests that occur rapidly enough such thata plurality of intermediate firing fractions are never actuallyattained, etc.

Torque Management

As suggested above, transitions are generally smoother when the torquedelivered during the transition matches the desired torque. One of theprime reasons for controlling the firing fraction in conjunction withthe air charge in the described manner is to help reduce torquevariations—which tends to help reduce undesired vibrations. When aircharge/firing fraction mismatches occur the output of the engine can bemodulated in other manners. One such approach is to control the sparktiming in a manner that mitigates such torque variations. Generally whenoperating at an allowed firing fraction level the spark timing is set ator near a timing that provides for optimum fuel efficiency, i.e. maximumtorque for a given MAC, typically denoted as the maximum brake torque(MBT) operating point. When the firing fraction increases and the aircharge is reduced, a torque surge would naturally occur when the firingfraction increases faster than the corresponding decrease in the aircharge. This surge can be mitigated by retarding the spark appropriatelyduring the transition in a manner that provides more steady torqueoutput. In general, retarding the spark can reduce the output of eachfiring as is well understood by those familiar with the art. If thespark timing prior to the transition did not correspond to the maximumtorque timing, the spark can be advanced a limited amount to provideslightly more torque per firing, although knock, misfire considerations,etc. typically limit the practicality of using spark advance. Therefore,the spark retard approach is particularly useful in avoiding a torquesurge. Such conditions exist when the firing fraction increases morequickly than the manifold can be emptied in a low to high firingfraction transition or if the manifold begins filling before a change inthe firing faction in a high to low firing fraction transition.Generally use of firing fraction or air charge delay can be used tomitigate the torque mismatch, allowing slightly more than requestedtorque (if uncorrected), which can be reduce by modest spark retard. Arelatively short firing fraction ramp, i.e. high slew rate, can then beused to reduce the mismatch between the firing fraction and air charge.The firing fraction slew rate can be defined either in terms of time orsome parameter based on engine speed, such as crank angle, firings orfiring opportunities.

An advantage of using spark timing control to help ensure that theengine provides the desired torque throughout the entire transition isthat the spark is easy to control and can be adjusted very quickly. Assuggested above, spark retard can be used to reduce the torque mismatchthroughout an increasing firing fraction transition. In some cases sparkretard alone may be sufficient to eliminate the mismatch; however, inother cases the air charge lag may be too great to be compensated byspark retardation without compromising combustion stability. In allcases an undesirable side effect of retarding spark to reduce engineoutput is that retarding spark will generally reduce fuel efficiency.Therefore, to the extent possible, it is generally preferable to matchthe air charge to the skipping fraction throughout the transition asdescribed above to avoid, or at least reduce, the fuel efficiency lossesassociated with spark retard control.

For comparison purposes, FIGS. 6(a)-6(d) illustrate the firing fraction,manifold pressure, spark advance and overall engine torque output duringan intended constant torque transition utilizing spark retard inconjunction with a delayed linear slew to further help mitigate thetorque surge. Like in FIGS. 3(a)-(d) and 4(a)-(d), the transition isfrom a firing fraction of ⅓^(rd) to a firing fraction of ⅔^(rd). Thechange in the throttle position is similar to that depicted in FIG. 3(b)and FIG. 4(b) and is not depicted in FIGS. 6(a)-6(d). In these figuresthe initial spark timing is optimized for maximum brake torque, the mostefficient operating point. FIG. 6(a) shows the change in the firingfraction 310 over the course of a firing fraction transition managedusing a delayed linear slew transition approach described with respectto FIG. 4. FIG. 6(b) shows the corresponding change in MAP 231 andskipping fraction 330 over the transition. The areas 332 a and 332 billustrate areas of mismatch between the skipping fraction 330 and theMAP 231. As shown in FIG. 4(d) these areas correspond to regions oftorque mismatch if no corrective measures are taken. FIG. 6(c)illustrates the change in the spark timing during the transition. If thespark timing is held at its maximum efficiency the result is curve 510.However, it may be desirable to deviate from this maximum efficiency sothat the torque surge can be eliminated. This type of adjustment isdepicted in curve 512. The spark is retarded at the beginning of thetransition to eliminate the torque bump seen in FIG. 4(d). FIG. 6(d)shows the resultant impact on torque. The spark adjusted torque 540shows no torque surge at the beginning of the transition due to thespark retardation reducing the per cylinder output. The torque dip 542near the end of the transition cannot be removed using spark timing,since the spark timing is already adjusted to its maximum efficiencypoint.

In some cases the torque dip 542 may be undesirable. In such casesreducing the delay T_(D) will result in the MAP 231 always being aboveand to the right of the skipping fraction 330 in the transition. In thiscase the engine would always be producing excess torque, which could beremoved by retarding the spark timing. While this improves NVH, it willreduce fuel efficiency. In other cases an engine may be generallyoperating with a spark timing somewhat different than the timing thatyields maximum efficiency, typically denoted as the maximum brake torque(MBT) point. In this case the engine has a torque reserve such that bycontrolling spark timing the engine torque can be both increased anddecreased. Operating an engine with torque reserve has the undesirableeffect of reducing fuel efficiency, so firing fraction transitioncontrol strategies described herein minimize the need to operate awayfrom MBT spark timing.

Some engine controllers have the ability to cut off the delivery of fuelto a cylinder while still opening the valves in the regular manner. Thistechnique causes intake air to be pumped through the cylinders andeffectively cuts the engine's output to zero. Currently, the most commonuse of fuel cutoff is during deceleration wherein fuel is typically cutoff from all working chambers—a practice commonly referred to asdeceleration fuel cutoff (DFCO).

During skip fire transitions that would otherwise result in a torquesurge (e.g., increasing firing fraction while decreasing air charge) asomewhat similar approach can be used to help balance the air chargewith changes in the firing fractions. Specifically, during skip firecontrol, selected working cycles are not fired. Generally, the cylindersassociated with skipped working cycles are deactivated so that they donot pump air through the cylinder during a skipped working cycle.However, if it is desirable to reduce the amount of air in the manifold,the valves associated with selected skipped working cycles can beactivated to pump air through the corresponding cylinders during theskipped working cycles. Since the working cycles are intended to beskipped, no fuel is delivered to the working chambers and no combustionoccurs.

The number of skipped working cycles that are appropriately used to pumpair through the engine block for any particular transition will varybased upon the nature of the transition. For example, factors such asthe intake manifold dynamics, the initial and target firing fractions,the initial and target air charges, the firing fraction slew rateutilized, the engine speed, the otherwise expected torque surge, etc.can all impact both the number of working cycles appropriately used topump air through the cylinder and their respective timing. Someadvantages of using the air pumping approach include that it can savefuel relative to other torque mitigation approaches (e.g. spark retard)and it helps speed the transition by helping more rapidly reduce themanifold pressure to the desired level. A potential drawback orlimitation of this approach is that the emissions system (e.g. catalyticconverters) must be able to handle the air passed through the engine andnot all emissions systems will have that ability all of the time.However, when practical, the use of skipped cylinders to pump excess airout of the manifold during firing fraction transitions can be asynergistic use of skipped working cycles during skip fire control.

Pumping air through some or all of the skipped cylinders has theadvantage of more quickly reducing MAP, thus allowing a fastertransition with potentially lower levels of NVH. FIG. 7 illustrates theMAP and possible skipping fractions in two cases, a case without pumpingair and a case of pumping air. The transition begins at time t₁. Thecase without pumping is equivalent to that previously discussed inrelation to FIG. 6(b). The skipping fraction 330 is substantiallymatched to the MAP 231 with relatively small mismatch areas 332 a and332 b. The case with pumping by the skipped cylinder has a much fasterdrop in the MAP 731 and consequentially the skipping fraction 730 canchange more quickly and still be substantially matched to the MAP 731.The mismatch areas 732 a and 732 b are similar in size to thoseassociated with the non-pumping transition 332 a and 332 b. The totaltransition time t_(pump) is much shorter than the previous transitiontime t_(tr). As seen in FIG. 2 shorter transition times are advantageousif NVH_(mismatch) can be held to an acceptable level, since they havelower NVH_(pattern) and thus potentially lower overall NVH. Undercertain conditions, the transition time can even be reduced to zero,i.e. the target fraction is reached in a step function transition on thefollowing working cycle.

An alternative method to adjust the filling/emptying of the intakemanifold is to change the intake and exhaust valve timing. For camoperated valves this is done by adjusting the cam phasor, which controlsthe relative timing of the valve opening and closing. For engines withvariable valve lift or electronically controlled valves greater controlis possible. In all cases valve motion may be adjusted to provide adesired MAC for a given MAP (within the system control range). Thisallows another degree of control during the transition. In some casesthe valve timing used during the transition may also be used at thefinal firing fraction level.

Feed Forward Air Control

As previous discussed in regard to FIGS. 3b and 4b an engine controllermay use feed forward control of the throttle to accelerate desiredchanges in manifold pressure. The example illustrated used a simple stepfunction change in the commanded throttle position to adjust the MAP.However, more complex control schemes on the throttle may be used toachieve a faster transition in the MAP. Some control schemes mayintegrate a feed forward control architecture with various types offeedback control, such as a PID (proportional, integral, differential)controller or state-space controller to better control MAP response. Ingeneral, feed forward throttle control contemplates opening or closingthe throttle more than would be appropriate for steady state operationduring the transition and then backing off to the level appropriate forsteady state operation. Using feed forward throttle control duringtransitions between different firing fractions can help accelerate thetransition in a controlled manner which can help further reducevibration. Since the transition occurs more quickly, a higher slew ratecan typically be used during such transitions.

Most of the discussion above has focused on the use of a throttle as theprimary mechanism to vary the air charge in each cylinder. As will beappreciated by those familiar with engine operation, there are other wayto vary the air charge as well including valve timing control,intake/exhaust valve timing and lift control, supercharging, etc. Wherepractical, feed forward control can be used to control the air chargeusing these air charge control mechanisms in addition to, or in place ofthe described feed forward throttle control. For example, if electronicvalves are provided, the opening and closing timing of each valve canreadily be controlled to facilitate more rapid transitions in a desiredair charge. When the valve train is controlled by one or more camshafts,feed forward control of the cams or camshafts can be used to facilitatemore rapid transitions of the air charge. Similarly, when the valvetrain supports variable valve lift, appropriate control of the valvelift (including feed forward control) can be used to better match theair charge to the firing density. When the engine includes suitablehardware, any of these air charge control mechanism can be used inparallel.

The described feed forward air control can be used independently or incombination with spark retard, and/or pumping air through deactivatedcylinders and/or the described firing fraction transition slew ratecontrol. A desirable characteristic of feed forward air control is thatit can be used in conjunction with both increases and decreases of thefiring fraction.

Although only a few embodiments of the invention have been described indetail, it should be appreciated that the invention may be implementedin many other forms without departing from the spirit or scope of theinvention. For example, the transition slew rate limiting has beendescribed primarily in the context of using constant linear slew ratesthroughout the transition. Although such an approach works well, itshould be appreciated that more complex slew rates can be used whendesired—which may be useful to better track specific manifold fillingand/or emptying dynamics and/or other design considerations. Forexample, in some implementations, it may be desirable to divide thetransition into two or more linear segments or to define a more complextransition function.

Some skip fire controllers are arranged such that they will inherentlyinvoke a relatively large number of transitions under a variety ofnormal driving scenarios in an effort to maximize fuel economy. This isparticularly true in driving conditions that support a relatively largeset of firing fractions. By way of example, some driving tests byApplicant of a skip fire controller having up to 29 available firingfractions tend to average a transition every second or two duringvarious normal driving profiles. For driving comfort, this makes itparticularly desirable to utilize some of the transition managementapproaches described herein.

Several different techniques including firing fraction management, airdelivery management and spark timing management have been described.Although each may be used independently, better results are oftenobtainable when used in combination with the goal of avoiding transitorytorque surges or dips while facilitating rapid transitions betweenfiring fractions.

In the foregoing description, there are several references to the term,“cylinder.” The term cylinder should be understood as broadlyencompassing any suitable type of working chamber. The figuresillustrate a variety of devices, designs and representative cylinderand/or engine data. It should be appreciated that these figures areintended to be exemplary and illustrative, and that the features andfunctionality of other embodiments may depart from what is shown in thefigures.

The invention has primarily been described in the context of dynamicskip fire operation in which an accumulator or other mechanism tracksthe portion of a firing that has been requested, but not delivered, orthat has been delivered, but not requested. However the describedtechniques are equally applicable to managing transitions between anydifferent skip fire firing fractions or between a skip fire firingfraction (in which individual cylinders are sometimes fired and sometimeskipped) and all cylinder operation (or operation using a fixed set ofcylinders) as may occur when using various rolling cylinder deactivationtechniques. Similar techniques may also be used to manage effectivedisplacement transitions in variable stroke engine control in which thenumber of strokes in each working cycle are altered to effectively varythe displacement of an engine.

The present invention may also be useful in engines that do not use skipfire control. For example, although the invention is described primarilyin the context of transitions between different firing fractions duringskip fire control, the described techniques can also be used tofacilitate transitions between different variable displacement states inmore traditional variable displacement engines using a skip firetransition approach. For example, an eight cylinder variabledisplacement engine that has the ability to operate in a 4 cylinder mode(i.e., 4 fixed cylinders) will require transitions from a firingfraction of 0.5 to 1 and vice versa and could advantageously use thefiring fraction transition management techniques described herein.Therefore, the present embodiments should be considered illustrative andnot restrictive and the invention is not to be limited to the detailsgiven herein.

The invention claimed is:
 1. A method of controlling the transition ofan engine between different firing fractions, the method comprising:while the engine is operating at a first firing fraction, determining asecond target firing fraction that is different than the first firingfraction; and transitioning from the first firing fraction to the targetfiring fraction by gradually altering a commanded firing fraction fromthe first firing fraction to the second firing fraction, wherein thecommanded firing fraction is altered each firing opportunity.
 2. Amethod of controlling the transition of an engine between differentfiring fractions, the method comprising: while the engine is operatingat a first firing fraction, determining a second target firing fractionthat is different than the first firing fraction; and transitioning fromthe first firing fraction to the target firing fraction by graduallyaltering a commanded firing fraction from the first firing fraction tothe second firing fraction, wherein the commanded firing fraction isaltered by substantially the same amount each firing opportunity.
 3. Amethod as recited in claim 1 wherein: the commanded firing faction hasan associated skip fraction which is a complementary fraction of thecommanded firing fraction; and the commanded firing fraction is alteredin a manner such that a product of the skipping fraction and the intakemanifold pressure remains substantially constant throughout thetransition.
 4. A method as recited in claim 1 wherein the firingfraction is changed at substantially the same rate throughout thetransition.
 5. A method as recited in claim 1 wherein the commandedfiring fraction is provided to a skip fire firing timing determiningmodule that includes an accumulator functionality that tracks a portionof a firing that has been requested, but not delivered, or that has beendelivered, but not requested.
 6. A method as recited in claim 1 whereinthe commanded firing fraction is changed each firing opportunity using alinear slew rate such that the amount that the commanded firing fractionis changed each firing opportunity is the same throughout thetransition.
 7. A method as recited in claim 2 wherein the commandedfiring fraction is changed each firing opportunity using a linear slewrate such that the amount that the commanded firing fraction is changedeach firing opportunity is the same throughout the transition.
 8. Amethod as recited in claim 6 wherein the linear slew rate is in therange of 1-5% such that the commanded firing fraction increases in therange of 1 to 5 percent each firing opportunity.
 9. A method as recitedin claim 6 wherein the magnitude of the linear slew rate is selected atleast in part based on the magnitude of the change in firing fractionand at least one engine operating parameter.
 10. A method as recited inclaim 1, wherein the transition period is in the range of 150 to 300milliseconds.
 11. A method as recited in claim 1, wherein the engineincludes a multiplicity of working chambers and an intake manifold thatsupplies air to at least a plurality of the working chambers, the intakemanifold having a manifold air pressure, the method further comprising:changing a commanded throttle position in conjunction with thetransition between different firing fractions to facilitate operation atthe target firing fraction, wherein initiation of the altering of thecommanded firing fraction is delayed relative to initiation of thechange in throttle position by a plurality of firing opportunity,thereby helping compensate for inherent delays associated with changingthe manifold air pressure.
 12. A method as recited in claim 1, whereinthe engine includes a multiplicity of working chambers and an intakemanifold that supplies air to at least a plurality of the workingchambers, the method further comprising: determining a target manifoldpressure associated with the target firing fraction, the target manifoldpressure being different than an initial manifold pressure that existswhen a decision to change firing fractions is made; and utilizing feedforward throttle control in conjunction with the transition toaccelerate the transition of the manifold pressure to the targetmanifold pressure.
 13. A method as recited in claim 1, wherein theengine includes a plurality of cylinders, a plurality of intake valves,each intake valve being associated with an associated one of thecylinders, a camshaft arranged to open and close the intake valves; andan intake manifold that supplies air to the cylinders through the intakevalves, the method further comprising: determining a target air chargeassociated with the target firing fraction, the target air charge beingdifferent than an initial air charge that exists when a decision tochange firing fractions is made; and utilizing feed forward camshaftcontrol in conjunction with the transition to accelerate the transitionof the air charge to the target air charge.
 14. A method as recited inclaim 1, wherein the engine includes a multiplicity of working chambers,each working chamber having an associated spark source, the methodfurther comprising: determining a target spark timing associated withthe target firing fraction, the target spark timing potentially beingdifferent than an initial spark timing that exists when a decision tochange firing fractions is made; and retarding the spark relative toboth the initial spark timing and the target spark timing for selectedfired working chambers during the transition to mitigate or prevent atorque surge that would otherwise occur during the transition.
 15. Amethod as recited in claim 14 wherein at least one of the initial andtarget spark timings is a spark timing that causes the engine togenerate the maximum brake torque at the associated engine settings. 16.A method as recited in claim 1 wherein: the engine includes an intakemanifold, an exhaust and a multiplicity of working chambers, eachworking chamber being arranged to operate in a succession of workingcycles; at least one of the first and target firing fraction has acomplementary skip fire firing fraction in which selected skippedworking cycles are not fired; air is pumped through the engine from theintake manifold to the exhaust during selected skipped working cyclesthat occur during the firing fraction transition to more quickly reducemanifold pressure during the transition; and wherein air is generallynot pumped through the engine during skipped working cycles that occuroutside the firing fraction transition.
 17. A method as recited in claim1 further comprising changing at least one commanded engine operatingparameter that affects a working chamber air charge in conjunction withthe transition between different firing fractions to facilitateoperation at the target firing fraction, wherein initiation of thealtering of the commanded firing fraction is delayed relative toinitiation of the change in the commanded engine operating parameter bya plurality of firing opportunity, thereby helping compensate forinherent delays associated with increasing or decreasing the amount ofair in an intake manifold that provides air to the working chamber. 18.A method of controlling the transition of an engine from an initialfiring fraction to a target firing fraction, there being an initialmanifold pressure and a target manifold pressure, the target manifoldpressure being lower than the initial manifold pressure and the targetfiring fraction being higher than the initial firing fraction, themethod comprising: operating the engine in a skip fire manner during thetransition; and pumping air through the engine from an intake manifoldto an exhaust during selected skipped working cycles that occur duringthe transition to more quickly reduce intake manifold pressure to thetarget manifold pressure; and wherein air is generally not pumpedthrough the engine during skipped working cycles that occur outside thefiring fraction transition.
 19. A method of controlling the transitionof an engine between different firing fractions, the engine including amultiplicity of working chambers, an intake manifold and an exhaust, theintake manifold having a manifold air pressure and being arranged tosupply air to at least a plurality of the working chambers, the methodcomprising: while the engine is operating at a first operational firingfraction, receiving a request to transition to a target secondoperational firing fraction that is different than the first operationalfiring fraction; transitioning from the first operational firingfraction towards the target second operational firing fraction bygradually altering a commanded firing fraction from the firstoperational firing fraction towards the target second operational firingfraction; and changing a commanded throttle position in conjunction withthe transition between the different firing fractions to facilitateoperation at the target second operational firing fraction, whereininitiation of the altering of the commanded firing fraction is delayedrelative to initiation of the change in throttle position by a pluralityof firing opportunity, thereby helping compensate for inherent delaysassociated with changing the manifold air pressure.
 20. A method asrecited in claim 19 wherein when a request is received to transition toa third operational firing fraction before the transition to the targetsecond operational firing fraction is completed, the method furthercomprises: changing the commanded throttle position to a throttleposition that facilitates operation at the third operational firingfraction in response to the request to transition to the thirdoperational firing fraction; continuing to transition towards the targetsecond operational firing fraction during a designated delay period; andafter the designated delay period has expired, transitioning from thethen current firing fraction to the third operational firing fraction bygradually altering the commanded firing fraction from the then currentoperation firing fraction towards the third operation firing fraction.21. A method as recited in claim 20 wherein the designated delay periodis a defined time period.
 22. A method as recited in claim 20 whereinthe designated delay period is a defined number of firings or firingopportunities.
 23. A method as recited in claim 19 wherein the commandedfiring fraction is altered each firing opportunity.
 24. A method asrecited in claim 19 wherein the commanded firing fraction is altered bysubstantially the same amount each firing opportunity.